Carbon--Carbon (C--C) composites are a specialty class of materials having many unique properties making them attractive for a variety of demanding engineering applications. Similar to many other high-performance composites, C--C composites consist of a continuous fiber reinforcement (graphite fiber) held within a matrix phase (carbon). Unlike other composites, both the reinforcement and matrix phases consist of essentially pure carbon. Demonstrated desirable properties of C--C composites are very lightweight (1.4-1.9 g/cc), low coefficient of friction, good fatigue and shock resistance, moisture resistance, no outgassing, good biocompatibility, radiation resistant, low coefficient of thermal expansion (anisotropic in most cases), excellent strength retention versus temperature, does not melt or soften to mention some of its properties. Undesirable properties include anisotropicy in thermal expansion, oxidation above about 425.degree. C., low interlaminar strength in two-directional (2-D) composite construction using graphite fabric reinforcement and very high cost of manufacture. These undesirable properties have largely limited major applications of C--C composites. Each of these limitations must be overcome for widespread applications.
In aerospace applications such as the space shuttle, missile nose tips and rocket nozzle throats, C--C composites have found applications in spite of the property limitations and high cost which ranges from a few hundred dollars per pound to as much as $15,000/lb. For C--C composites to achieve success outside the specialty applications, their cost of manufacture must be substantially reduced. The high cost of C--C composites is directly related to the lengthy processing times and energy intensive manufacturing procedures used to make component shapes. Raw materials of graphite fibers (depending on the fiber type) and resin or pitch matrix precursors cost are generally relatively low. The matrix infiltration and densification processes are both time consuming and energy intensive, and therefore, costly.
There are several reinforcement architectures used in state-of-the-art C--C composites. The most often architecture is woven graphite fabric laid-up as laminates (2-D). There are also 3-dimensional and 4-dimenstional woven structures that are considerably more expensive than cloth or fabric weaves. Other reinforcement architectures include multifilament threads referred to as tows, chopped discontinuous tows or fabrics, felts and other discontinuous reinforcement forms. Regardless of the reinforcement architecture, the C--C composites are produced by resin or pitch impregnation followed by cure (thermosetting) and pyrolysis. Since the carbon yield from resins or pitches is generally limited to less than 50-70%, multiple cycles of impregnation and pyrolysis is required as practiced, prior to the present invention, to achieve a reasonable density (low porosity) composite, the density of the composite is directly related to the composite mechanical properties. In the prior methods to reduce the porosity, reimpregnation followed by further pyrolysis with the cycle repeated up to eight times is required to reduce the porosity to an acceptable level of under 10%. However, typically the matrix precursors pyrolize to result in closed porosity which cannot be reimpregnated and with these prior methods it is nearly impossible to produce a C--C composite with a porosity of less than about 20-15%, even with repeated reimpregnations.
The high cost of C--C composites are directly related to the hand lay-ups of 2-D composites to near net-shapes and the multiple matrix densification cycles often requiring 4-6 cycles. The laminates are characteristically thicker and subsequently heavier than required to carry-in-plane loads because the 2-D woven laminates have relatively low (a few hundred psi) interlaminar shear and cross-ply strengths. Thus, the parts must be thicker to maintain the interlaminar stresses below the allowable stress levels which adds to both the materials and manufacturing costs. Some approaches to overcome the interlaminar strength limitations is to utilize braiding, knitting or cross stitching, which is expensive and is known to degrade the graphite fibers in the 2-D orientations from the abrasion as well as reduce overall composite fiber loading and in-plane properties. Multidirectional 3-D, 4-D, etc. architectures improve interlaminar properties but are substantially more expensive to weave and the structures are processed as billets followed by extensive machining resulting in very expensive finished components.
In order to achieve a universal application of C--C composites, a significantly more cost effective fabrication process must be demonstrated that produces acceptable mechanical properties and particularly overcomes the low interlaminar properties of the 2-D type composites. To achieve these objectives, the fabrication processing must produce a net shape without the use of labor intensive hand lay-ups, only one impregnation-cure-pyrolysis densification step is permissible that produces a high composite density (low porosity) with mechanical properties that carry the loads in the intended application, the interlaminar shear is at least a factor of two above state-of-the-art 2-D C--C composites, and the anisotropicy is virtually eliminated. These are formidable objectives which have been attempted by many others heretofore without success in the aggregate or in combinations. As will become obvious, the instant invention achieves these objectives for C--C composites that can lead to widespread applications such as components in internal combustion engines; for example, pistons or thin plates for thermal management.
One of the most formidable obstacles is one-step net shape molding without labor intensive operations. Comparatively, economical net shape one-step molding is accomplished with discrete metal or ceramic particles, or chopped fiber glass. These reinforcements are mixed with a resin binder and net shape molded in a state-of-the-art segmented die, shaped bladder, autoclave, etc. with curing in the die. Such molding is not possible with continuous graphite fibers, 2-D woven cloths or 3-D type architectures as the fibers buckle and become askew resulting in very poor mechanical properties. This is the reason labor intensive hand lay-ups have been utilized. Discontinuous or chopped graphite fibers have been utilized to produce composites analogous to chopped fiber glass which can be economically one-step molded to net shape. However, traditionally discontinuous graphite fiber C--C composites have resulted in very poor mechanical properties of one-fourth or less than 2-D fabric composites. As an example, U.S. Pat. No. 4,683,809 column 3, line 47, reports 8ksi (55 MPa) strength for random oriented composites. Such results have been similarly obtained by the applicants, and in commercial literature, for example, 10-80 ksi, even lower mechanical properties are reported for discontinuous graphite. Generally, there are no discontinuous graphite fiber reinforced C--C composites utilized in commercial applications because of the poor mechanical properties. Thus, although economical net shape molding of discontinuous reinforcements could be utilized to alleviate labor intensive hand lay-ups, mechanical properties are insufficient to permit commercial utilization.
Even though net shape molding can be economically achieved by known techniques using discontinuous graphite fibers and a resin or pitch binder or even small pieces of impregnated cloth cut into geometries of, for example, 1/4.times.1 inch, 1/2.times.1/2 inch, etc., when the binder/carbon matrix precursor is pyrolyzed, the carbon yield is sufficiently low that a porous low density matrix results along with unacceptably low mechanical properties. The composite density of a 50 vol % graphite fiber-50 vol % matrix composite will generally be much less than 1.4 g/cc due to porosity in the matrix as a consequence of low carbon yield of the resin or pitch matrix precursor.
To overcome this low density/high porosity matrix, several reimpregnations and pyrolysis steps are required to increase the density to acceptable values of 1.5-1.8 g/cc, which substantially improves the mechanical properties. To increase the carbon yield of the resin or pitch, attempts in the past have included adding various fixed carbon such as carbon black particulates, calcined cokes or graphite powder. A problem from such additives is an increase in the viscosity of the liquid resin or pitch resulting in poor impregnation of the matrix precursor. It is also common that after pyrolysis, the matrix is poorly bonded and powdery as a result of the fixed carbon additives and particularly carbon black. This is in part due to the high surface area of carbon black particulates. Another problem is the graphite reinforcement is typically 7 to 10 micron fibers in the form of tows or fabric architectures and the particulate fillers do not penetrate the fiber tow bundles or fabrics. Thus porosity remains in the fiber tows or fabrics including their interticies after pyrolysis resulting in poor mechanical properties.
An alternative to increasing the carbon yield of any organic is to reduce volatilization during pyrolysis, that is prevent carbon species from evaporating during pyrolysis. If an overpressure is utilized, then the vapor pressure of any volatile component is reduced. If, for example, the carbon yield of a phenolic resin were 60% under atmospheric pressure, if the pressure were increased to above atmospheric, then the carbon yield should increase to above 60%. The higher the pressure, the higher the carbon yield up to the point that only hydrogen would be lost from the hydrocarbon.
Certain organic structures in a resin/polymer results in less hydrocarbon loss than other structures. For example, polyaryacetylene (PAA) has a higher char yield of 75-90% depending on heating rate and surface area. The volatile loss in PAA is only methane and hydrogen which accounts for higher char yields. The effect of the organic composition and structure of the resin as well as pressure of pyrolysis and heating rate can have a direct and substantial effect on char yield that directly effects the residual porosity of the carbon matrix.
An alternative process of producing C--C composites is the pyrolysis of a hydrocarbon gas in a graphite reinforcement array which is called chemical vapor infiltration (CVI). The CVI process entails isothermally pyrolizing a hydrocarbon gas in the reinforcement array, creating a thermal gradient in the reinforcement array to pyrolize the hydrocarbon gas and build up the carbon matrix or forced flow of the hydrocarbon gas through the reinforcement array to build up the carbon matrix. A disadvantage of the variations of the CVI process is it is very slow requiring many hundreds of hours to build-up the carbon matrix, small pores are filled in preference to large pores and the deposition of the carbon seals off pores resulting in 30 to 10% porosity. A major disadvantage is that the surface deposition forms a continuous layer sealing off and leaving a porous core. To overcome this disadvantage typically only regular shaped parts are produced such as brake linings which are machined on the surface to open the pores followed by additional deposition. The machining to open pores and redeposition cycle is often repeated up to five or six times which is quite similar to the resin or pitch reimpregnation process. Thus, in the prior art method it is difficult to produce a C--C composite with less than 15-10% porosity which if achieved requires surface machining and reimpregnation.
One of the major limitations in C--C composite applications in addition to residual porosity is poor interlaminar shear strength. Much work has been reported in the art on graphite fiber surface treatment and composite processing to improve the interlaminar properties of shear and tensile strength. In general, very little success has been achieved for improving the interlaminer properties. An approach to increasing interlaminer properties in C--C composites is to whiskerize the graphite fiber reinforcement. Whiskers growing off the surface of a graphite fiber acts like a barb and prevents shear failure at the graphite fiber interface. The whiskerizing can also alter the surface chemistry of the graphite fiber that can affect the interlaminar shear properties of C--C composites. Whiskerizing graphite reinforcements has been known in the art but after whiskerizing the strength of the graphite fiber was reduced to such a low level it would be unusable to produce composites. Whiskerizing has typically involved producing silicon carbide whiskers.
It is known that gaseous silicon oxygen compounds react with carbon to produce silicon carbide and carbon monoxide. The temperature, pressure and concentration of reactants determine whether a solid carbon surface is converted to a layer of silicon carbide, whiskers are nucleated on the surface of the carbon or silicon carbide. Various methods of generating silicon oxide gases such as SiO.sub.2(g) of SiO.sub.2(g) are utilized to react with carbon bodies to form a silicon carbide layer. Such reactions are widely utilized to produce oxidation resistant coatings on carbon objects, and many attempts have been reported to convert graphite fibers to silicon carbide fibers. Utilizing the teachings of U.S. Pat. Nos. 3,459,504; 3,385,723; 3,447,952; 3,371,995; 3,269,802; 4,900,531; 6,634,116; 4,596,741; 4,554,203; 4,476,178; 4,481,179; 4,513,030; 3,580,731 and GB 2,147,891A graphite fibers converted to silicon carbide had unusable strengths and were, in some cases, severely cracked whether a thin silicon carbide layer was formed or the entire cross sections of the graphite fiber was converted to silicon carbide. Although the teachings from heretofore produced silicon carbide surfaces and/or silicon carbide whiskers, the product was unusable because of its very low strength. For example, T300 graphite fiber from Hercules as received has a tensile strength of 3-5 GPa and its diameter is approximately 7 microns. After converting to silicon carbide, the average tensile strength was less than 1/2 GPa, which is so low that it is unusable. T300 graphite fiber was whiskerized as taught in U.S. Pat. No. 3,580,731 and the strength of the whiskerized fiber was also less than 1/2 GPa and in some cases, unhandable with a strength less than 0.1 GPa. Even if only 10% of the cross section of the T300 fiber is converted to silicon carbide, the strength of the resultant fiber is less than 1 GPa and in some tests, less than 1/2 GPa, which is too weak for any reasonable application. Other commercially available graphite fibers from polyacrynialnitriel (PAN) and pitch were equally degraded when subjected to conversion to silicon carbide and/or whiskerization utilizing state-of-the-art teachings.